1. Field of the Invention
The present invention relates in general to measurement-while-drilling systems, and in particular to improved antennas for use in measurement-while-drilling systems.
2. Description of the Prior Art
The efficient and problem-free communication of data has been one long-standing goal of the oil and gas industry. The communication of data is particularly important during drilling operations, but is also of interest in completion and production operations. Two principal techniques exist for efficient and reliable communication: the communication of data utilizing a mud pulse telemetry system, and the communication of data utilizing electromagnetic transmissions. Of course, high quality antennas are required in order to transmit and receive electromagnetic energy at a deep subterranean location. Even for other transmission systems, such as the mud pulse telemetry data transmission system, it turns out that reliable antennas are also desirable, due principally to the fact that logging operations are frequently performed during drilling operations, utilizing antennas to transmit interrogating electromagnetic waves into the borehole and surrounding formation, and then utilizing antennas to detect the amplitude attenuation or phase shift of the interrogating electromagnetic wave.
Antennas, whether used for the electromagnetic communication of data or for the transmission and reception of interrogating fields during logging operations, are rather delicate devices which cannot be too heavily shielded or they will not be able to perform their functions. But they are so delicate that they cannot be exposed to wellbore conditions, particularly during drilling operations, without substantial risk of harm. Consequently, most art antenna constructions utilize solid wellbore tubulars, such as drill collar tubulars and drill pipe tubulars, to form a housing which protects the antenna from damage due to the corrosive fluids, high pressures, and high temperatures frequently encountered in wellbores particularly during drilling operations. The most common prior art technique requires that a portion of the tubular be "necked-down" during milling and/or machining operations by radially reducing the tubular at a particular location to provide a rather deep and wide groove. Typically, a layer of cushioning and electrically-insulating material is provided in the groove, and the antenna windings are wound about the tubular at the position of the groove to protect the antenna from physical damage to allow the communication of electromagnetic fields between the antenna windings and the borehole and surrounding formation, a slotted sleeve was typically provided, and secured in position over the antenna windings provided within the necked-down portion of the tubular member.
For the most part, this type of construction provided a satisfactory result; however, the wellbore tubular is weakened in the region of the necked-down portion, to an extent that may not be acceptable in particularly difficult drilling operations. Additionally, since the antenna windings are disposed beneath at least one substantial layer of steel presented by the slotted sleeve, additional electrical energy is expended in order to provide for adequate signal levels.